The invention relates generally to hydraulic lifting devices and more specifically relates to a hydraulic lifting device for use in connection with harvesting machines having a wheel frame and a harvesting apparatus such as a mowing table supported on the frame by a hydraulic cylinder-piston unit in such manner that the harvesting apparatus rests on the ground with its residual weight only. The hydraulic device is of the type having a multi-way control valve connected between a pressure fluid pump and a tank and having ports for selectively connecting by a first working pipeline the cylinder-piston units to a lifting or lowering position, and further including hydraulic accumulator and a locking valve unit for the cylinder-piston unit.
Conventional lifting devices of this type have a pressure limiting valve to safeguard a hydropneumatic energy accumulator and a locking valve for protecting the working position of the cylinder-piston unit, the locking valve unit being connected in series with a working pipeline leading from a multiway control valve to the cylinder-piston unit. In spite of the fact that the prior art hydraulic device insures a tight locking of the lifting cylinder-piston unit and during the lowering of the harvesting apparatus inhibits a complete discharge of the energy accumulator, it still has the disadvantage that the lowering process runs relatively slowly because the entire pressure fluid flow has to pass through the pressure limiting valve. Due to the fact that most of the weight of the harvesting apparatus has to be supported by the lifting pressure cylinder unit of the device, the pressure limiting valve is adjusted to such a valve that it is relatively close to the working pressure in the cylinder and consequently the pressure difference causing the discharge of the pressure fluid stream is relatively low. A further disadvantage is in that the pressure limiting valve when operating in proximity of its reference pressure does no longer perfectly seal and pressure fluid can leak into subsequently connected pipelines as far as to another closed valve seat.